1. Field of the Invention
This invention relates to a round or flat insulated wire used as magnet wires for various kinds of electric coils in electric and electronic equipment.
2. Description of the Related Art
In recent years, combined with down-sizing and weight reducing tendency for equipment in various fields such as in the automobile and electric or electronic industries, the demand has grown for reduced size and weight of coils used in such equipment as well as low cost manufacturing while maintaining high performance such as electric characteristics, mechanical characteristics and long lasting heat resistance. Therefore, it is necessary for forming coils to wind magnet wires around a smaller core at a higher density and at a higher speed, which results in damaging the insulation film for the magnet wires, thereby deteriorating the electromechanical characteristics of equipment or lowering the production yield.
For coping with the problems, countermeasures have been taken for the insulation film of insulated wires such as (1) improvement of mechanical strength, (2) improvement of flexibility, (3) improvement of lubrication and (4) improvement of adhesion with conductors. For example, Japanese Patent Application Laid-Open No. 196025/1994 (a) describes the technique of improving the resistance to fabrication of a heat resistant insulation film made of polyamideimide, polyimide or aromatic polyamide by properly setting the tensile strength, tensile modulus of elasticity, adhesion and static friction coefficient to piano wires. Further, Japanese Patent Application Laid-Open No. 58519/1987 (b) discloses a flat insulated wire formed by coating and baking an enamel prepared by adding a polyisocyanate block material blocked with a polyesterimide and a phenolic compound to a polyetherimide. Further, Japanese Patent Application Laid-Open No. 34828/1983 (c) discloses a blend of a polyamideimide and a polyetherimide, to obtain a material having mechanical characteristics comparable with those of the polyetherimide and excellent in solvent resistance, abrasion resistance and long lasting heat resistance.
However, in the technique disclosed in (a), while the insulation film is constituted taking into consideration the improvements (1), (3) and (4), the flexibility is not always sufficient. In the technique disclosed in (b) the problems of degradation due to fabrication and lowering of thermal impact resistance of insulation film caused by rolling an insulated wire to have a flat square shape by combining the flexibility of the polyetherimide, the heat resistance and the adhesion of the polyesterimide to the conductor, and the solderability of phenolic compound-blocked polyisocyanate, but the film does not have sufficient flexibility since elongation at the break of the polyesterimide is inadequate (for example, refer to Comparative Example 7 in this specification). Each of the blend materials obtained in (c) has low glass transition temperature (refer to Table 2 of the patent publication (c)), and the heat resistance is not sufficient to be used as a heat resistant magnet wire material.
As has been described above, although the related art as in (a), (b) and (c) described above can solve the given subjects respectively as the magnet wire material, they are not yet completely satisfactory for obtaining further excellent winding fabricability while maintaining high heat resistance. It may be attributable to the provision of different characteristics which is intended in any of the prior arts is done basically by combining various materials in a single layered structure.